Greasemonky script Gmail Attachment Reminder gives you a gentle nudge if it looks like you've forgotten to attach a file when the email you've just written suggests that those were your plans.

The short of it: If you write an email containing the word "attach" or pretty much any variation thereof (attachment, attached, etc.) but you don't have an attachment present, this handy-dandy little script will pop-up an alert box to remind you not to make that mistake yet again. Way to go, Greasemonkey! — Adam Pash